The present invention relates to an apparatus used in medical ultrasound imaging. In particular, it relates to an apparatus which uses multiline beamforming.
The term "multiline beamforming" refers to a method of image reconstruction wherein multiple receive lines are used for each transmit line. The obvious advantage of multiline beamforming is the potential for obtaining high frame rates with a given line density. In particular, by reconstructing multiple, simultaneous receive lines for each transmit line, it is possible to obtain frame rates equivalent to the multiple of receive lines generated. For example, by reconstructing three simultaneous receive lines for each transmit line, frame rates of approximately 120 frames per second with 128 lines per scan at a range of 15 centimeters could be generated. This approximation is based upon a velocity of propagation of approximately 1.5 mm/.mu.sec. Cardiac scans obtained at this speed and viewed at a lower rate could reveal details of motion which would aid in the diagnosis of certain clinical problems, such as valvular disease and septal defects.
Speckle reduction could be obtained at the same frame rate as is presently used. Averaging frames obtained under different conditions, such as aperture, frequency, and zone modulation, could be preformed, and the results could be displayed at different frame rates. Such averaging could improve image quality significantly. Also, it might be possible, at high frame rates, to view the motion of blood by following the speckle pattern produced by the scattering of red blood cells. Using this method, viewing would be performed on frames replayed at a slower frame rate than the rate at which they were obtained. Similarly, this method could be used to speed up the doppler flow imaging scan rates. Presently, it is necessary to slow the frame rate down to rates of 4 to 10 frames per second to acquire doppler data for a ninety degree sector. Multiline beamforming offers the potential for increasing this rate significantly.
While the idea of multiline beamforming has been around for some time, heretofore, there has been no practical way to implement a device employing multiline beamforming without degrading image quality.
During simulations performed previously to determine the effect on image quality of this method of imaging, it was discovered that image quality was degraded. It was postulated that the reason for the poor image quality was the narrow width of the transmit beam. Conventional methods of widening the beam, such as decreasing the transmit aperture and focusing at large distances resulted in loss of resolution with very little improvement in image texture.
Skipping transmit lines introduces artifact into the image. The larger the angular spacing between transmit lines, the more apparent the artifact will be. The increased artifact results from the decreased number of samples as a function of angle. Accordingly, the image, which contains special frequencies in excess of those which can be reproduced without artifact at the given sample rate, is undersampled. Such undersampling of the space domain signal, as a function of angle, introduces artifact in a manner similar to undersampling a time domain signal in time.
One way to limit the artifact would be to limit the spacial freqency content which can be processed. Unfortunately, the processing occurs in the array beam, and the only control of the array beam is via altering the acoustic beam. Widening the beam averages the spacial frequency content, so it would be expected to reduce artifact. Again, by analogy, widening the beam is equivalent to low pass filtering a time domain signal.
Assuming that reciprocity holds in the space domain, the effect of widening the transmit beam without altering the receive beam should be the same as the effect of widening the receive beam without altering the transmit beam. In practice, however, the effect has been found to be somewhat different if the receive beam is dynamically focused. Two relatively simple, but crude, methods of widening the transmit beam are to decrease the aperture on transmit and to move the focal point far out so that the beam is wider than where the aperture is focused at the desired image plane. In practice, moving the focus point out is quite limited due to the natural focusing qualities of a finite aperture transducer. It has been shown experimentally that both moving the focus point out and decreasing the aperture size are effective in reducing artifact. However, reducing the aperture has a more predominant effect on artifact and produces less resolution degradation. Reducing the aperture also reduces the penetration.